LogFAQs > #979838756

I don't know if the later games carried the concept forward, but there were rare monsters in the Phantasy Star Online games. If you weren't specifically running quests with large numbers of the common variants, it would take weeks or months of playtime until you spotted one.

In Final Fantasy XI, a lot of "Notorious Monsters" would be a chance of spawning instead of one specific enemy in the zone (I.e. there are 6 flies in this area, one of the six is the placeholder and Valkurm Emperor may spawn in its place). But there's no guarantee people will be killing the placeholder to even give it a chance to spawn, so it might become incredibly rare to actually see.